The present invention relates to forming thin film transistors, particularly to forming silicon thin film devices on plastic substrates, and more particularly to forming a thick adherent dielectric film intermediate a plastic substrate and a silicon thin film device which protects the plastic substrate during laser processing of the device.
Development efforts are underway for the fabrication of silicon thin film transistors, for example, on plastic substrates to produce lightweight devices. These development efforts have been focused on manufacturing polycrystalline silicon (poly-Si) based thin film transistors at low temperatures on plastic substrates. Two technical hurdles that must be overcome are to manufacture and to dope a poly-Si film while preventing any thermal damage to the plastic substrate, which generally cannot withstand sustained temperatures over about 200xc2x0 C. Conventional processes to produce or dope poly-Si require sustained temperatures at or above 600xc2x0 C., a temperature range that will damage plastics.
By the use of pulsed laser annealing to produce poly-Si, as described and claimed in U.S. Pat. No. 5,817,550, issued Oct. 6, 1998 to Paul C. Carey, et al., which involves using a high intensity ultraviolet excimer laser pulse that is absorbed at the surface of the silicon film, thermal damages to most plastic substrates was significantly reduced. The pulse energy is sufficient to heat the silicon film to its melting point (xcx9c1400xc2x0 C.), but the time period of the pulse was short so as not to significantly heat the plastic substrate.
Efforts to fully resolve the above-referenced hurdles were directed to the formation of a thermal barrier layer between the silicon film and the plastic substrate, which serves to dissipate the heat so that the plastic substrate is not exposed to high temperatures that would damage the plastic. These efforts, for example, involved a conventional PECVD deposition technique to deposit a film of silicon dioxide, SiO2, at 100xc2x0 C. for use as the thermal barrier. The thicknest film that could be achieved using this method was 0.75 xcexcm. Films greater than 0.75 xcexcm deposited by this method would easily crack when subjected to bending, and indicated very high stresses. Efforts to deposit thicker non-cracking films by varying the deposition conditions failed. Following this, commercially available materials were sought from the film coating industry. Thick (2-4 xcexcm) films of commercially available VITRINITE, made by MetroLine, were deposited on plastic substrates. While use of these thermal barrier layers resulted in few cracks during the silicon processing, they immediately cracked and delaminated when submersed in water, a necessary requirement for the poly-Si processing. Other commercially available films also delaminated and cracked when exposed to water. As the result of the unavailability of thermal barrier films, and since the above-referenced 0.75 xcexcm SiO2 film was not sufficiently thick to fully protect the plastic substrate during poly-Si processing, even utilizing pulsed laser processing, there was a need for a thermal barrier film having a thickness of 1-5 xcexcm, preferably xcx9c4-5 xcexcm, and with the following properties; 1) adheres to plastic substrates, 2) does not lift-off when cycled in temperature, 3) has no cracks and does not crack when subject to bending, 4) resistant to lift-off when submersed in fluids, 5) electrically insulating, and 6) preferably transparent.
The present invention provides a solution to the above-referenced technical hurdles and provides thick adherent dielectric films having the above-listed properties which can be composed of various materials and deposited on the plastic substrate by various known deposition techniques. This is accomplished by cooling the plastic substrate during deposition of the thermal barrier layer to near room temperature by use of a cooling chuck. The method for producing the thick (1-5 xcexcm) adherent dielectric film includes directing cooling gases between the plastic substrate and the cooled substrate retaining chuck.
It is an object of the present invention to provide a plastic substrate with an adherent thick (1-5 xcexcm) dielectric film.
A further object of the invention is to provide a method for depositing adherent dielectric films on plastic substrates which includes cooling the substrate during deposition of the films.
Another object of the invention is to provide a plastic substrate with a thermally protective film to enable processing of components at temperatures that would normally damage the plastic substrate.
Another object of the invention is to deposit thermal barrier layers on plastic substrates and deposit material to be processed on the thermal barrier layer, whereby the plastic substrates are protected from high temperatures which occur during laser annealing of the material deposited on the thermal barrier layer.
Another object of the invention is to provide a thermal barrier layer of plastic substrates which adheres to the plastic substrates, has a thickness of 1 xcexcm or greater, does not lift-off when cycled in temperature, has essentially no cracks and does not crack when subjected to bending, is resistant to lift-off when submerged in fluids, is electrically insulating, and is preferably transparent.
Other objects and advantages of the present invention may become apparent from the following description and accompanying drawings. The invention involves a thick, adherent, dielectric film deposited between a plastic substrate and one or more layers of material to be processed at temperatures that would damage the plastic substrate but for the dielectric film. The film may have a thickness of 1-5 xcexcm, preferably 4-5 xcexcm and may be formed from a variety of dielectrics by a variety of deposition techniques. Films deposited using the process of this invention can be used for a variety of applications, including but not limited to: use as a thermal barrier layer for laser annealing, use as a permeation/diffusion barrier, as a field dielectric for the electrical isolation of devices, a planarization layer, scratch resistant layers, and as a thermal/electrical insulation layer between device layers in multilevel circuitry. By the deposition of the thick dielectric layer, composed for example of SiO2, between a plastic substrate, such as PET, by PECVD, and one or more layers of polycrystalline silicon, the sustained temperature of 600xc2x0 C. or above, (pulsed temperature in the silicon layer of 1400xc2x0 C.) conventionally utilized a laser annealing process or to dope the polycrystalline silicon, does not damage the plastic substrate in that the dielectric layer dissipates the heat so that the plastic substrate is not exposed to high temperatures. The deposition of the dielectric film is carried out using active cooling of the plastic substrate wherein the substrate is placed on a cooling chuck or mandrel and a cooling gas is directed between the cooling chuck and the substrate, such that, for example, the plastic substrate is maintained at about room temperature during deposition of the thick (1-5 xcexcm) dielectric film.